Did you uncomment the following lines of code on ProductDetails.ascx?

<%--Lightbox Live Server--%>

<%--<link href="/css/lightbox.css" rel="stylesheet" type="text/css" />

<script type="text/javascript" src="/js/prototype.js"></script>

<script type="text/javascript" src="/js/scriptaculous.js?load=effects,builder"></script>

<script type="text/javascript" src="/js/lightbox.js"></script>--%>

In order for the lightbox to work, you need to uncomment the code so it looks like this:

<%--Lightbox Live Server--%>

<link href="/css/lightbox.css" rel="stylesheet" type="text/css" />

<script type="text/javascript" src="/js/prototype.js"></script>

<script type="text/javascript" src="/js/scriptaculous.js?load=effects,builder"></script>

<script type="text/javascript" src="/js/lightbox.js"></script>

Thanks,

Christian